Most of what I learned about music was from a sight singing course in High School that everyone in Chorus had to take to make life easier for the coral director. We looked at the sharps and flats in the signature and that told us where "DO" (as in DO RE MI)was on the musical staff. No sharps or flats and "DO" was C. But if the signature had no sharps or flats and the melody started down on "LA" (A) and ended there we found ourselves singing in A minor. Try it. Sing DO TI LA descending and then sing LA TI DO RE etc up to LA and then back down to LA and I hear a minor scale. (Which type of minor scale - DUH)